"Connectivity" here needs to be taken at large: syncthing needs to have a folder already shared between two machines. There's some setup involved that is not automatic.
The entire setup phase of magic wormhole is "copy those 3 words" and boom you're done.
I think a lot of people talking about Synchthing must have used it in a different way in the past. It has a QR code now that encodes that 16-digit number or whatever that is used for the same purposes of key exchange and initial handshaking.
I use it with my wifes phone to transfer files between her drawing tablet and her Linux system she uses for Blender every day.
Yes, but even that is more involvement: you need to create a new specific folder, add a device, share that folder, accept on the receiver, copy the file in the folder, and wait for full sync in the web ui. It's a much longer setup that makes sense if you will synchronize files/folders with the same machine in the same folder, but that's not always the case. magic-wormhole removes all that back-and-forth dance for one-off sends.
The entire setup phase of magic wormhole is "copy those 3 words" and boom you're done.